"Your test method isn’t conclusive. Rather than evaluating the differential while the transmission is in neutral, you should always perform this evaluation with the transmission in gear. This prevents the ring gear from spinning, and will force the differential to spin the opposing wheel in the opposite direction when you turn one by hand.
Let me back up a step – the gearing of a differential (any differential: open, LSD, torque-biasing, etc) requires the average speed of the two axle shafts & wheels to be equal to the speed of the differential input (the ring gear). So, when the ring is prevented from turning, if you turn one shaft forward, the other must turn in the opposite direction. It’s basically the same as saying that the average of +1 and -1 must equal 0. So, by the same token, if opposite wheel cannot turn when you turn a wheel/axle, then either the ring gear must turn (at half the speed you turn the wheel) or you will not be able to turn the wheel at all. The geared mechanical connection forces this action.
So, back to your test. Often, if the transmission is in neutral, there is more inertia and drag to overcome in the opposite wheel and brake assembly then there is in the transmission gearing, which isn’t connected to the clutch, so there isn’t anything to prevent it from turning. As such, the ring gear will often turn inside the gearbox instead of having the opposite wheel turn. I see this sort of thing occur all of the time. As a result, you cannot conclude that the differential is faulty until you perform this test with the transmission in gear.
For the moment, I would speculate that there is nothing wrong with the differential, especially if you see no sign of damage or breakage within the assembly. When you reassemble the unit, torque the cap bolts to 55 lb-ft and use Locktite. Pay attention to the orientation and location of each gear to be certain that they are go back to the same places that they came from, in the same way. It is easiest to reassemble all of the internal parts on the cap, then slide the housing down over top of it. If you have any questions, don’t hesitate to ask.
Best regards,
Rick Barnes
JTEKT Torsen NA Incrbarnes@torsen.com

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PTE 6262CEA
Pump Gas 385whp 319wtq
Race Gas 551whp 439wtq
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